This is an amazing, for the sole reason that everyone who is 17 and change now will turn 18, be able to smoke, the law will bump to 19, they won't be allowed to smoke any more, but then they'll turn 19 and they'll be able to smoke again until the law raises to 20...
This is the better way to write the law of course, but the ham-fisted way it's proposed by Rishi would look more like what I wrote, because he said specifically that the age should rise one year every year.
So the age is going to get risen to 19, then the Tories are going to lose the next election (basically the only reason to have the election at this point is to find out by how much they're going to lose so we know how much to laugh at them), and then the law stops getting updated because it's a dumb and badly written, and then Labour don't implement it any more.
If anything they will probably just rewrite the law to the above version.